Luminol is a good tool as a last resort to keep in our box of stuff.
For anyone who doesn't know what luminol is, it's a blood finding agent used in police forensic investigations. The stuff is pretty amazing. It finds microscopic blood residue that cannot be seen with the naked eye and it also finds blood that's been rained-out. In fact, it works even better after rain. You mix 2 tablets in water, or better yet...windshield washer fluid. Spray it on the suspected area after dark, use no lights. When luminol is sprayed on blood it will glow blue for a couple minutes. We've used it on test blood trails and even after a half inch of rain and ZERO visible blood, the trail glowed brightly.
It's not a gimmick. The stuff works but there is a learning curve to using it. I consider it a last resort tool. Hunters should still rely on the basics...ethical shots, sharp heads, and solid blood trailing skills. But sometimes stuff happens.
Luminol obviously can't find blood that isn't there. If the deer isn't bleeding externally, it can't help.
For about $25 you can get enough that (they claim) will trail ~400 yards, but it really would depend on how liberally you are spraying the mix.
This is where I bought mine...
blood glow.com